you can register any accout by post /auth/login/sentry

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

If you change `op=login` to `op=register`, you can register any account.

### Expected Result

fix it

### Actual Result

you can register any accout by post /auth/login/sentry

### Product Area

Unknown

### Link

_No response_

### DSN

_No response_

### Version

_No response_

Start by tracing POST /auth/login/sentry and comparing how it handles op=login versus op=register. Verify the reported unauthenticated registration path and identify the intended authentication behavior before changing it. Done means unintended account creation through this endpoint is prevented without breaking the intended login flow; no file or test is named.

Written by the indexing model from the issue text.